Most traditional constant voltage power supplies are designed to minimize output impedance in an attempt to simulate an ideal voltage source. The present invention relates to a power supply having an adjustable equivalent output resistance, which can be either positive or negative. Positive equivalent output resistance can be useful in the simulation of batteries where internal resistance is a critical parameter. Alternatively, negative equivalent output resistance can be utilized to greatly improve voltage regulation at the load in the situations where the voltage sense leads are located a distance from the load itself.
The present invention provides a power supply that is capable of producing a negative or positive equivalent output resistance. In accordance with the preferred embodiment of the present invention, the equivalent output resistance can be adjusted in such a way that it transitions smoothly between positive and negative values.
Power supplies can be used to simulate a battery. This is useful to battery powered device manufacturers who require that all interactions between the device and its battery be properly tested before the unit is shipped. The battery, which is electrochemical in nature, tends to degrade over time as it is discharged and recharged. Other factors, such as thermal cycling, may also impair the performance of the battery. Using a power supply in place of the battery allows the tester to capture critical performance data about the operation of the device under test. To achieve results that closely mimic that of an actual battery, the power supply must closely match the battery""s output resistance and voltage characteristics. As the battery of the device ages, degradation in its performance is caused by an increase in the internal resistance of the battery. Consider a mobile (cellular) telephone. When the phone attempts to transmit and link up, it draws a substantial amount of current, which causes the battery voltage level to drop. If the voltage drops below a critical level, the telephone call will be terminated. With age, the increase in the battery""s internal resistance results in larger current draws, bigger voltage drops, and an increased number of terminated calls. Therefore, manufacturers are interested in simulating the battery resistance to better characterize these products. Hence, having the flexibility to adjust the equivalent positive output resistance of the power supply can be of particular importance.
Alternately, some manufacturers are not interested in simulating the battery resistance characteristics and are instead interested in maintaining a constant voltage at a specific load point under varying load current conditions. Utilizing remote sense leads, the voltage of the power supply can be precisely controlled at the point where the sense leads are attached. However, it is not always possible to connect the sense leads directly to the load, possibly because of mechanical interference or some other reason. As shown in FIG. 1, an additional resistance, RL2, is found in the conducting path between the sense leads and the load, resulting in an undesired voltage drop. A power supply capable of generating a negative output resistance could solve this problem by compensating for the voltage drop caused by the resistance found after the sense leads. As a result, the voltage level supplied to the load could be accurately controlled. However, to date, such a power supply has not been produced.
Accordingly, a need exists for a power supply that is capable of generating a negative equivalent output resistance. A need also exists for a power supply that is capable of generating either negative or positive equivalent output resistances. Furthermore, a need exists for a power supply that is capable of smoothly transitioning between negative and positive equivalent output resistances. The present invention achieves these goals, as will be apparent from the following discussion.
The present invention relates to a power supply capable of being configured to produce a bipolar output resistance, i.e., either negative or positive output resistances. The electrical circuitry of the power supply is capable of being configured to produce a negative output resistance. In accordance with the preferred embodiment of the present invention, the electrical circuitry of the power supply is configured to produce either a negative or positive output resistance.
In addition, in accordance with the preferred embodiment, the electrical circuitry of the power supply is configured to enable continuous transitions to be made from negative resistance values through zero to positive resistance values, and vice versa. Preferably, the power supply comprises a multiplier chip that enables the continuous transitions to be achieved. Components other than the multiplier chip can be utilized to achieve a negative output resistance and to enable the power supply to switch between negative and positive output resistances, as discussed below in greater detail.
In accordance with this embodiment, the multiplier chip receives a reference voltage VREF that can be varied in magnitude and polarity in order to change the output resistance of the power supply. The reference voltage for the multiplier chip can be provided by either a potentiometer or a digital-to-analog converter capable of producing a bipolar analog voltage. Selecting an appropriate negative output resistance allows the power supply to effectively cancel the voltage drop caused by load wire resistance (RL2 in FIGS. 1 and 2) between the sense points and the load. This allows the voltage level provided to the load to be accurately maintained at the desired set value. These and other features of the present invention will become apparent from the following descriptions, drawings, and claims.